Research is proposed for the duration of 3 years to evaluate the thermal pulse-decay method of local tissue blood flow measurement. This new and conceptually innovative thermal method of perfusion measurement promises to offer solutions to many of the technological and methodological problems which have constrained the use of other tissue blood flow measurement techniques. The main objectives are to: (1) develop the effective use of a minimally traumatic, pulse-heated, thermistor bead microprobe for measuring local tissue blood flow; (2) use 1-6 microprobes with multiple probings to map the in vivo blood flow distribution in selected organs i the anesthetized dog and/or rabbit or in the in vitro control-perfused organ; (3) integrate the local perfusion measurements to obtain the total tissue flow and compare that value to independent total flow measurements made with an electro-magnetic flow meter; (4) compare and contrast the laboratory observations of in vivo and in vitro perfusion measurements with an analysis of perfusion in modeled tissues possessing angioarchitectures ranging from one with a completely homogeneous and isotropic character, to one having a very structured anisotropic vascular pattern: interpretation of the results will draw heavily upon an examination of the microvascular contribution to the partial differential equation of heat transfer in perfused tissue. The thermal pulse-decay technique of measuring local tissue blood flow has application in numerous areas of cardiovascular study where an acute or chronic evaluation of the normal, intended therapeutic, or pathological alteration of tissue blood flow is of concern. Quantitative blood flow measurements can be obtained in tissues which have proved difficult to evaluate in the past, e.g. kidney medulla, and liver. In addition, the thermal-decay method gives for the first time the opportunity to examine the role of the vasculature on the mechanisms of heat transfer in tissue.